This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. Primary support for the subproject and the subproject's principal investigator may have been provided by other sources, including other NIH sources. The Total Cost listed for the subproject likely represents the estimated amount of Center infrastructure utilized by the subproject, not direct funding provided by the NCRR grant to the subproject or subproject staff. Chlamydia trachomatis causes the most bacterial sexually transmitted diseases in the US. A comprehensive understanding of the observed pathogenesis caused by this bacterium is lacking. Innate immune cells such as macrophages are able to kill chlamydia. However, recent findings characterize macrophages into functionally different groups such as classically activated (CA) macrophages with potent anti-microbial activities and alternatively activated (AA) macrophages with a lack of anti-microbial activities. We found that Chlamydia trachomatis can grow in AA macrophages and that co-culture of AA macrophages overcame growth restriction of chlamydia in CA phagocytes. It is hypothesized that anti-chlamydial effector mechanisms of CA macrophages are attenuated by exposure to AA macrophages. In order to test the hypothesis we aimed to 1) examine expression and activity of anti-chlamydial effectors such as indoleamine dioxygenase (INDO) and GTPases in CA macrophages compared to those attenuated by co-cultured AA macrophages and 2) identify attenuating paracrine factors secreted by AA by assessing up-regulation in their gene expression. The macrophage line THP-1 was tested as a substitute model for the primary human macrophages originally proposed. We determined that resting THP-1 cells allowed chlamydial inclusion growth in immunofluorescence assays, classically activated THP-1 cells inhibited it - similar to primary macrophages. Likewise, PCR analysis showed induction of INDO-1 and GTPase GBP-2 message in RT-PCR for CA THP-1 cells. A novel molecular target, BLIMP-1, that was reported to suppress INDO expression was found in our hands to be up-regulated in CA m[unreadable]. In conclusion, THP-1 cells can be used to study attenuated CA m[unreadable]. B lymphocyte induced maturation protein-1 (BLIMP-1) is hypothesized to be a novel target.